Usage/History: The alidade is used with a plane table
in making convenient and direct surveys. Plane table surveying
has continued in use since the late sixteenth century due to
its simplicity. Initially a simple rule with a couple of plain
sites served as an alidade. In use the plane table is mounted
on a ball joint for leveling and oriented with a compass, frequently
part of the alidade as in this example. A sheet of paper is secured
to the table and an initial staion indicated on the paper. A
sighting is made with the alidade and the line of sight is marked
on the paper. The table is then moved to another station at a
measured distance, oriented with the compass, and a second sight
line is taken and drawn. Sighting the same targets results in
intersections on the paper cooresponding to the targets.
Some contemporary/early descriptions of the alidade and its
use are provided below:
Description: An oval brass tag, attached with brass
pins to the top of the mahogony case, is stamped: HUMBOLDT S-T-C
/ 555.
Bibliolgraphy:
Kiel;y, Edmond R. Surveying Instruments: Their History.
Carben Surveying Reprints, Columbus (1979).
Smart, Charles E. The Makers of Surveying Instruments
in America Since 1700. Regal Art Press, Troy (1962).
